This is an ink sensor for an ink jet array and more specifically is a comparator circuit which uses the discharge time of the capacitance of the ink in the channels of an ink jet array, and an on chip capacitor, to sense the presence or absence of ink.
In an ink jet printer, if the jet array runs out of ink for any reason, the operator usually discovers it by noticing that the output pages are blank. Unless the operator is continually watching the output tray, an amount of time and wasted output may result.
The situation is more critical in a color copier which uses multiple colors to generate the output. It could easily happen that one of the three, four or more color ink jet arrays will run out of ink, but the printer would still be producing images, and the operator may not detect that the output is off-color. By the time the problem is discovered, the output of multiple runs may have been delivered to the customers. Nor can the output be saved. In most printers the colors must be laid down in the correct order. A color that is not deposited at the proper cycle in the printing process can not be put in later.
One solution is to place an ink sensor in the channel that leads from the main ink reservoir to the array. Thermisters have been used to detect the heat conduction characteristics of the surrounding fluid, the ink having a greater thermal conductivity that the air would have in the absence of ink. One problem with this apparatus is that, over time, deposits form on the thermister which cause it to give an erroneous output.